Calorimetry procedures are often performed on chemicals and mixtures of chemicals to evaluate thermal hazards and other safety concerns in chemical manufacturing, shipment, storage and handling procedures. Thermal instabilities and uncontrolled exothermic reactions can lead to a rapid release of energy which can manifest itself in cataclysmic explosions and other dangerous and destructive situations. Consequently, it is important to identify in advance as much information as possible about particular chemical reactions and related behavior of chemicals and mixtures of chemicals under various manufacturing, storage, and handling conditions.
Chemical reactions can be described by physical parameters such as temperature, pressure, concentration and time. Proper hazard assessment can be accomplished only when these parameters and the consequences of possible deviations of these parameters have been well investigated and determined. One of the safety hazards of most concern in the chemical industry is the thermal hazard, which is governed by the thermodynamics and chemical kinetics taking place in the often complex chemical reactions induced. For example, if there is an exothermic reaction taking place, the relative rate of heat production and resulting temperature range of heat build-up is critical. Additionally, if there is pressure build-up in the system, it is imperative that the vessel containing the chemicals be of sufficient strength and design to withstand maximum pressures and avoid rupture or other damage.
Various techniques have been devised to address these concerns. One such technique is known as Differential Scanning Calorimetry (DSC) in which the heat of reaction and various reaction kinetics can be evaluated. Conventional DSC techniques, however, are unable to measure pressure of the chemical reactants, and are generally poor at elucidating low activation energy or autocatalytic processes. In thermal hazard evaluation, reactions involving the release of gaseous products are often encountered. Conventional DSC devices often include standard aluminum sample pans which cannot withstand pressure build-up. Such pressure increase may damage or rupture the sample pans and/or the analyzer during the experiment, and further hampers the quantitative evaluation of the DSC data.
Another technique initially developed by the Dow Chemical Company is known as Accelerating Rate Calorimetry (ARC). The ARC technique comprises maintaining a sample in an adiabatic condition once an exothermic reaction is detected. In this way, the heat generated from the reaction will accelerate the reaction. While ARC techniques will obtain both pressure and thermal readings, the execution of the adiabatic requirement is very difficult, the equipment and experiments are expensive and difficult to maintain, and the testing requires several grams of sample to achieve the accuracy desired.
Conventional thermal analysis apparatus have generally utilized substantially solid furnace block elements to provide both support and heat to a sample vessel located therewithin. In these arrangements, a sample compartment or recess to receive a sample vessel is provided in a solid block of aluminum, silver, or similar material highly conductive of heat. Contact between the heated block and the sample compartment enables radial conduction of heat inwardly to the chemicals to be tested. Closely surrounding the sample compartment with a block of high thermal mass, however, tends to interfere with the sensitivity of the apparatus to temperature changes, thereby compromising the reliability and accuracy of the thermal analysis device.
A modified calorimeter device marketed by System Technik Ag. (Systag) under the trademark Radex contemplates supporting a sample vessel by way of a plurality of glass centering cylinders stacked upon one another within a cylindrical jacket heater. In this arrangement, a sample is to be warmed via thermal resistance (i.e., by air surrounding the sample vessel) by controlled heating of the jacket heater. The difference between the sample temperature and the cylindrical jacket temperature is monitored to provide thermal analysis results. Sensitivity to heat flow variations, and precise control of the thermal energy applied to the sample being tested, are consequently dependent upon the relatively low conductive qualities of air.
Another device known as the Reactive System Screening Tool (RSST) marketed by Fauske & Associates of Burr Ridge, Ill. is described as enabling the determination of heat flow data and pressure data at a relatively low price. However, the glass test cell of this device is designed as an open vessel, which makes pressure measurements on condensible samples virtually impossible. Pressure measurements are further obtained and analyzed by components located relatively remotely from the sample vessel itself, and a reference tube is not provided within the heated zone of the device.
Moreover, conventional thermal analysis equipment and processes contemplate stand-alone devices and/or equipment which is designed to operate substantially independently of other devices. For example, while some devices, such as the DSC 111 Transducer, from Setaram, of France, are described as being "complimentary" to conventional DSC analysis technology, they generally comprise stand-alone equipment which is not designed to be utilized as a "plug-in" module which can be readily used with commercially available data acquisition systems. Moreover, the DSC 111 measures pressure externally of the heated zone, is subject to condensation problems, and is relatively expensive. Similarly, while devices such as the RCl Reaction Calorimeter System from Mettler Instrument (Hightstown, N.J.) include components described as "modular", these components are generally merely optional parts of an independently operated analysis arrangement.
Consequently, heretofore there has not been available a simple, inexpensive, reliable and accurate differential thermal analysis calorimeter which could readily be utilized as an ancillary plug-in device with commercially available thermal analysis data acquisition systems. Further, prior devices failed to provide a low cost, relatively simple arrangement which would enable accurate simultaneous measurement of heat flow of sample reactants and direct measurement of sample pressure during a reaction. Additionally, DSC type devices were limited to maximum sample sizes of about 50 mg, while ARC devices required at least several grams of sample for accurate results. Consequently, there was a gap in available technology, and a single device could not be used to handle a wide range of experiments.